President Barack Obama - who sought to “reset” U.S. relations with Russia when he took office - now faces the greatest foreign policy challenge of his presidency.

A standoff with Russia evoking memories of the Cold War is scarcely the global legacy that Obama has sought. He’s labored to end U.S. involvement in the wars in Iraq and Afghanistan and has proved reluctant to engage militarily in Syria, preferring diplomacy to military force. His administration has looked to rebalance its focus to emerging power economies in Asia.

But it’s the showdown with Russian President Vladimir Putin that may prove defining for Obama.

“By any standard this is the most difficult, the most complex international crisis he’s faced,” said former NATO ambassador Nicholas Burns, a former U.S. undersecretary of state for political affairs and member of the Atlantic Council Board of Directors. The dilemma in Ukraine cuts to the heart of decades-old American interests, Burns said: victory in the Cold War, stamping out Communism and overseeing the emergence of stable Europe.

“If Putin gets away with launching a military offensive, then Europe risks being divided,” Burns said. “The stakes are very high.”

Obama came into office with the belief that the United States needed to take a different approach to Russia than his predecessor. He maintained that George W. Bush’s administration had been too negative, continually “poking Russia in the eye,” said Stephen Larrabee, distinguished chair in European security at the RAND Corp.

Bush, for example, backed a Europe-based missile defense system that Russia opposed and pushed for NATO membership for Ukraine and Georgia, both former Soviet republics.

The Obama doctrine garnered some benefits, administration officials say: a strategic arms agreement, as well as cooperation from Russia on helping to curb Iran’s nuclear ambition and the delivery of supplies to U.S. troops in Afghanistan.

But Russia, which at the outset was “perfectly happy” to go along with Obama’s reset, became disillusioned when officials perceived the reset as forcing Russia to go along with the U.S. instead of having its views taken into account, said Anton Fedyashin, a Russia expert who directs the Initiative for Russian Culture at American University in Washington.

“There were high hopes for the reset but it didn’t really go well,” Fedyashin said. “The expectations were unrealistic. It was a lofty goal.”

The administration didn’t pay as much as attention to Russia as Russians thought it should, Fedyashin said.

“It’s not as important to the United States,” he said and it affects Russian pride.

The relationship was headed south by the end of 2011 when then Secretary of State Hillary Clinton called for a “full investigation” of irregularities in Russian’s parliamentary elections.

“The life went out of relationship in 2011,” said Andrew Weiss, who served as a former Ukraine and Russian expert in the Clinton White House and is now vice president for studies at the Carnegie Endowment for International Peace, a Washington research center.

Russia has long eyed the West warily, believing that Western nations, including the United States, took advantage of the country’s weakness after the Cold War, Weiss said: “They thought we took advantage of that,” he said.

Russia doesn’t put much stock in its relationship with the United States, and has watched the U.S. struggle with wars in Iraq and Afghanistan: “They’re not interested in good relations with us,” Weiss said.

The unraveling of the ties accelerated when Putin was returned to the presidency in 2012. He sat out the Obama-hosted Group of Eight meeting of leading industrial nations at Camp David in May 2012, instead sending Prime Minister Dmitry Medvedev – a move widely interpreted as a snub.

Obama and Putin tried to portray some bonhomie when they appeared together at a summit in Mexico a month later, but Putin, who has resisted Obama’s efforts to dislodge Syrian President Bashar Assad from power, sat expressionless as Obama talked about what Putin called “the Syria affair.”

Yet even as the relationship deteriorated, Obama has downplayed Russia as a global threat, poking fun at his Republican presidential rival Mitt Romney for labeling the country the U.S.’s “biggest geopolitical foe” on the campaign trail.

Russia, Obama joked to Romney at the third presidential debate in October 2012, is “calling to ask for their foreign policy back because, you know, the Cold War’s been over for 20 years.”

But it was Obama who invoked the Cold War last August after he scrapped a meeting in Moscow with Putin over the White House’s growing frustration with the Russian government over its embrace of intelligence leaker Edward Snowden, as well as its reluctance to engage in a host of issues.

“There have been times when they slip back into Cold War thinking and a Cold War mentality,” Obama said of the Russians on “The Tonight Show.” “And what I consistently say to them, and what I say to President Putin, is ‘That’s the past, and we’ve got to think about the future, and there’s no reason why we shouldn’t be able to cooperate more effectively than we do.’”

Earlier this year, Obama delivered a clear rebuke to Putin’s anti-propaganda laws that are widely viewed as anti-gay, sending a delegation to the Olympics in Sochi, Russia, that included openly gay members – and no high ranking administration figures.

But most Western leaders, including Obama, had underestimated Russia’s desire to reassert its influence in the Crimean region of Ukraine, the Rand Corp’s Larrabee said. Russia’s interests in Ukraine are far more important to Putin than his relationship with the United States, Larrabee said.

“Putin has made clear his objectives. He wants to restore Russia to a position of authority and respect,” Larrabee said. “You have to understand the resentment. This is personal. It’s the feeling that Russia is not going to be pushed around anywhere.”